1. Field of the Invention
The invention relates to the field of semiconductor filters, correlating means and recursive filters particularly employing integrated circuits.
2. Prior Art
In general, there has been a recognition of the benefits of semiconductor (integrated circuit) transversal and recursive filters. Such filters permit easy correlations (or convolution) between signals, and permit computation of time-invariant linear transforms. Among some of the specific suggested applications for such filters are: (1) determination of complex linear chirps for performing the discrete Fourier or cosine transforms by the Chirp-Z-transform algorithm, (2) broadband phase shifting for single sideband systems, and (3) broadband differentiators and integrators for synthesizing sampled data control loops, in addition to countless other signal processing problems. See "Signal Processing Architectures Using Transversal Filter Technology", by Whitehouse, Means and Speiser (Naval Underwater Center), Symposium on Circuits and Systems, 1975; and "The Serial Analog Processor" by Gene P. Weckler (Reticon Corporation), ISSCC 75, (Feb. 13, 1975).
Two general approaches have been employed for such filters or correlators. In one, an analog signal is stored while weighted taps are moved past the stored (static) signal. In the other approach, an analog signal is moved past fixed (weighted) taps. This latter approach permits real-time processing of an input signal.
In filters or correlators which employ a "moving" analog signal, most typically charge-coupled devices (CCD) or bucket-brigade devices (BBD), are used to transfer charge representing the input (analog signal). BBD may be realized as integrated circuits with both bipolar and MOS technology, and CCD may be realized with MOS technology. However, one major problem in obtaining a practical system has been the problem of providing a high impedance, weighted tap, to non-destructively sense the "moving" analog signal in a CCD or BBD register. The prior art solutions to this problem have, at best, one of the following shortcomings: (1) sensitive to normal semiconductor process variations such as oxide thicknesses, mask misalignment, etc. (2) require generation of special masks for each correlating function or signal, or (3) use expensive, non-integrated, discrete components. See "Bucket-Brigade Transversal Filter" by Puckette, Butler and Smith, IEEE Transactions on Circuits and Systems, Vol. CAS-21, No. 4, July 1974 (pages 502-510) for a discussion of "on chip" tap weights; also see "Transversal Filtering Using Charge-Transfer Devices: by Buss, Collins, Bailey and Reeves, IEEE Journal of Solid-State Circuits, Vol. Sc-8, No. 2, April, 1973 (pages 138-146). The techniques described in these articles include electrode weighting (split electrode) for CCD, weighted capacitors, and a transconductance technique.
Digital weighting of the outputs from a plurality of filters has been disclosed, see Charge Transfer Devices by Sequin and Tompett, published by Academic Press, Inc. (1975), Pages 230-231.
As will be seen, the present invention in its preferred embodiment, employs a novel bucket-brigade register for moving an input analog signal past taps which are easily (electrically) programmed. The summed or differenced output associated with each set of programmed taps is weighted, thereby permitting a highly resolved output signal in one embodiment to provide a transversal filter. For the most part, the filter is fabricated on a single MOS chip.